1. Field of the Invention
The present invention relates to a data recording medium, a recording apparatus and to a recording method for recording information to this data recording medium.
2. Description of Related Art
Data recording devices for optically recording information, and particularly digital data, to a storage medium are commonly used as a convenient means of mass data storage.
Phase change optical discs are one type of optical data recording medium. To record to a phase change optical disc a semiconductor laser emits an optical beam to a spinning disc to heat and melt a recording film on the disc. The achieved temperature and the cooling process (rate) of the molten film can be regulated by controlling the power of the laser beam, thereby inducing a phase change in the recording film.
When laser power is high, the recording film cools rapidly from a high temperature state and thus becomes amorphous. When a relatively low power laser beam is emitted, the recording film cools gradually from a medium high temperature state, and thus crystallizes. The resulting amorphous parts of the recording film are commonly known as "marks," and the crystallized part between any two marks is known as a "space." Two-value binary information can thus be recorded using these marks and spaces. When a laser beam is emitted at a high power setting to form a mark, the laser is referred to as operating at "peak power." When the laser is emitted at low power to form a space, the laser is said to operate at a "bias power" level.
During data reproduction, a laser beam is emitted at a power level low enough to not induce a phase change, and its reflection is then detected. Reflectivity from an amorphous mark is normally low, and is high from a crystalline space. A read signal can therefore be generated by detecting the difference in light reflected from the marks and spaces.
Data can also be recorded to a phase change disc using a mark position recording method (also known as PPM) or a mark edge recording method (also known as PWM). Mark edge recording normally achieves a higher recording density.
Mark edge recording typically records longer marks than recorded by the mark position recording method. When a laser emits at peak power to a phase change disc, heat accumulation in the recording film results in the mark width increasing radially to the disc towards the end part of the mark. In a direct overwrite recording method this can result in part of a mark not being overwritten or completely erased, resulting in a significant loss of signal quality due to signal crosstalk between tracks during reproduction.
Recording density can also be increased by shortening the lengths of the recorded marks and spaces, Thermal interference can occur when the spaces, in particular, are shortened beyond a certain point. This thermal interference can result in heat at the trailing edge of a recorded mark travelling through the following space, thus affecting heat distribution at the beginning of the next mark. Heat at the beginning of one recorded mark can also travel back through the preceding space and adversely affect the cooling process of the preceding mark. When such thermal interference occurs with conventional recording methods, the positions of the leading and trailing edges can shift, thus increasing the error rate during data reproduction.
Addressing this problem, Japanese Unexamined Patent Application Publication (kokai) H07-129959 (U.S. Pat. Nos. 5,490,126 and 5,636,194) teaches a recording method whereby a signal for forming a mark in mark edge recording is analyzed into three parts, a constant width beginning part, a middle part having pulses with a constant period, and a constant width end part, and this signal is then used to drive recording by rapidly switching the output of a two-value laser beam during mark formation.
With this method, the width of the middle part of a long mark is substantially constant and does not spread because laser output is driven with a constant period pulse current producing the minimum power required for mark formation. Jitter at the leading and trailing edges of the mark also does not increase during direct overwrite recording because a constant width laser beam is emitted to the leading and trailing end parts of the mark.
It is also possible to detect whether a mark or spaces before and after a mark is long or short, and change the position al which the leading and trailing parts of a mark are recorded according to the length of the mark and the leading and trailing spaces. This makes it possible to compensate during recording for peak shifts caused by thermal interference.
Japanese Patent Application 5-279513 does not, however, teach a method for determining the optimum positions of the leading and trailing parts of a mark.
If the method of optimizing the leading and trailing edge positions is not defined, the reliability of the optimized recording will be low. Furthermore, even if optimized recording is achieved, it will be at the expense of excessive time spent searching for the optimum position and excessive circuit cost.
A method for changing the leading and trailing edge positions of a mark based on the data being recorded has also been invented as a means of achieving high density data recording. A problem with this method, however, is that the edge of a recorded mark can move due to thermal interference as described above. Such edge movement is also highly dependent upon the disc format and the makeup of the recording film, and if either of these change even slightly, optimized recording cannot be achieved.
With consideration for the above described problems, an object of the present invention is to provide a method and apparatus for easily determining the optimum positions the leading and trailing edges of each mark, thereby achieving optimized recording, even when the disc format, recording film composition, and recording apparatus characteristics vary.